Salient features of a parallel programmable interface-8255, Computer Engineering

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Determine the salient features of a parallel programmable interface, 8255.

24 I/O lines in 3 8-bit port groups - A, B, C
A, B can be 8-bit input or output ports
C can serve as 2 4-bit input or output ports
3 modes of operation:

- Mode 0: A, B, C simple input or output level sensitive ports

- Mode 1: A, B input or output ports with strobe control in C

- Mode 2: A is bidirectional with control/handshake in B and C

A, B can only change 1 byte at a time
C has individual bit set/reset capability
Advantage is non-dedicated circuit can change port configuration with software and no 'glue logic'
Ports A, B, and C are used for I/O data.
The control register is programmed to select the operation mode of the three ports A, B, and C.
Mode0: simple I/O mode
Any of the ports A, B, CL and CU can be programmed as input or output.
No control of individual bits (all bits are out or all bits are in)
Mode 1: Ports A and B can be used as input or output ports with handshaking.
Mode2: Port A can be used as bidirectional I/O port with handshaking

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